The desirability of identifying or discovering new antimicrobial agents is widely recognized. New antimicrobial agents are desired for several reasons; these include, but are not limited to, development of microbe strains resistant to known antimicrobials, undesirable interactions of certain known antimicrobials with the medium or product in which the antimicrobial is used, and high toxicity of certain known antimicrobials to certain non-target organisms such as mammals.
Certain polyvalent iodine compounds are known to be effective antimicrobial agents. These known antimicrobial compounds are salts (see, for example, U.S. Pat. Nos. 4,440,943 and 4,513,137).
Iodones are iodonium ylide compounds that are a class of compounds of which little is known. It has now been discovered that certain iodones are iodonium ylides are effective antimicrobial agents.
In his book entitled Ylid Chemistry (A. William Johnson, Ylid Chemistry, Academic Press, New York and London, 1966, pp. 1-4), A. W. Johnson defines an ylid (typically spelled `ylide` in the current literature) "as a substance in which a carbanion is attached directly to a heteroatom carrying a high degree of positive charge". He further states that "this definition is intended to include those resonance hybrid molecules in which there is an important contributing structure which meets the original definition". There is a clear distinction between onium salts and onium ylides. In an onium salt, the positive charge on the heteroatom (i.e., a non-carbon atom) is balanced by the negative charge of a counteranion electrostatically associated with the onium ion (i.e., organic cation). In an onium ylide, the positive charge on the heteroatom is balanced by the negative charge of a localized or delocalized carbanion covalently bound to the onium center. Some examples are given below: ##STR1##
Notice that the protonation of an ylide with a strong acid would give an onium salt. Conversely, the treatment of an onium salt with a strong base would give an ylide; e.g., ##STR2##
Ylides may sometimes be referred to as "internal or inner salts", "betaines" or "zwitterions". For example, iodonium ylides derived from phenols have been described as:
ylides: P. B. Kokil and P. M. Nair, Tetrahedron Lett. 4113-4116 (1977) PA0 zwitterions: S. Spyroudis and A. Varvoglis, J. Chem. Soc. Perkin Trans. I 135-137 (1984). PA0 inner salt: S. W. Page et al., J. Am. Chem. Soc., 101, 5858-5860 (1979). They qualify as ylides because of resonance structure II below: ##STR3## PA0 n represents an integer from 0 to 2; PA0 Y represents a functional group capable of stabilizing the positive charge on the polyvalent iodine by a proximal nonbonded electrostatic interaction: ##STR8## represents a cyclic 1,3-dione anion: and their hydrates and alcoholates. PA0 "steric hindrance. A characteristic of molecular structure in which the molecules have a spatial arrangement of their atoms such that a given reaction with another molecule is prevented or retarded in rate."
Iodonium ylides derived from .beta.-dicarbonyl compounds are typically called betaines. For example, see B. Y. Karele and O. Y. Neiland, J. Org. Chem. USSR (Engl.), 2, 1656-1658 (1966).
Such terms are synonymous and more general than the term "ylide" and include all organic molecules in which a positive charge center is internally compensated by a negative charge center. However, while it is true that all ylides are internal salts, betaines and zwitterions, it is not true that all internal salts, betaines and zwitterions are ylides. Examples of internal salts (betaines, zwitterions) that are not ylides are shown below. They are not ylides because the negative charge center is neither carbanionic nor directly bound to the onium center. ##STR4##
Sometimes ylides are represented in such a way that they appear, at first glance, to be betaines but not ylides. A case in point is the representation of "phenyldimedonyliodone" as an enolate betaine. ##STR5##
However, phenyldimedonyliodone may be properly regarded as an iodonium ylide since a complete electronic description of the molecule must include the resonance structure below in which a carbanion is directly bound to the iodonium center. ##STR6##